The heart is a muscular pump whose mechanical activation is controlled by electrical stimulation generated at a right atrium and passed to the entire heart. In a normal heart, the electrical stimulation that drives the heart originates as action potentials in a group of pacemaker cells lying in a sino-atrial (SA) node in the right atrium. These action potentials then spread rapidly to both right and left atria. When the action potential reaches an unactivated muscle cell, the cell depolarizes (thereby continuing the spread of the action potential) and contracts. The action potentials then enter the heart's conduction system and, after a short delay, spread through the left and right ventricles of the heart. It should be appreciated that activation signals are propagated within the heart by sequentially activating connected muscle fibers. Each cardiac muscle cell generates a new action potential for stimulating the next cell, after a short delay and in response to the activation signal which reaches it. Regular electrical currents can be conducted in the heart, using the electrolytic properties of the body fluids, however, due the relatively large resistance of the heart muscle, this conduction cannot be used to transmit the activation signal.
In a muscle cell of a cardiac ventricle, the resting potential across its cellular membrane is approximately −90 mV (millivolts) (the inside is negatively charged with respect to the outside). FIG. 1A shows a transmembrane action potential of a ventricle cardiac muscle cell during the cardiac cycle. When an activation signal reaches one end of the cell, a depolarization wave rapidly advances along the cellular membrane until the entire membrane is depolarized, usually to approximately +20 mV (23). Complete depolarization of the cell membrane occurs in a very short time, about a few millisecond. The cell then rapidly (not as rapid as the depolarization) depolarizes by about 10 mV. After the rapid depolarization, the cell slowly repolarizes by about 20 mV over a period of approximately 200-300 msec (milliseconds), called the plateau (25). It is during the plateau that the muscle contraction occurs. At the end of the plateau, the cell rapidly repolarizes (27) back to its resting potential (21). Different cardiac muscle cells have different electrical characteristics, in particular, cells in an SA node do not have a substantial plateau and do not reach as low a resting potential as ventricular cells.
In the following discussion, it should be appreciated that the exact mechanisms which govern action potentials and ionic pumps and channels are only partly known. Many theories exist and the field in is a constant state of flux.
The electrical activity mirrors chemical activity in a cell. Before depolarization (at resting), the concentration of sodium ions inside the cell is about one tenth the concentration in the interstitial fluid outside the cell. Potassium ions are about thirty-five times more concentrated inside the cell than outside. Calcium ions are over ten thousand times more concentrated outside the cell than inside the cell. These concentration differentials are maintained by the selective permeability of the membrane to different ions and by ionic pumps in the membrane of the cell which continuously pump sodium and calcium ions out and potassium ions in. One result of the concentration differences between the cell and the external environment is a large negative potential inside the cell, about 90 mV as indicated above.
When a portion of the cell membrane is depolarized, such as by an action potential, the depolarization wave spreads along the membrane. This wave causes a plurality of voltage-gated sodium channels to open. An influx of sodium through these channels rapidly changes the potential of the membrane from negative to positive (23 in FIG. 1A). Once the voltage becomes less negative, these channels begin to close, and do not open until the cell is again depolarized. It should be noted that the sodium channels must be at a negative voltage of at least a particular value in order to be primed for reopening. Thus, these channels cannot be opened by an activation potential before the cell has sufficiently repolarized. In most cells, the sodium channels usually close more gradually than they open. After the rapid depolarization, the membrane starts a fast repolarization process. The mechanism for the fast repolarization is not fully understood, although closing of the sodium channels appears to be an important factor. Following a short phase of rapid repolarization, a relatively long period (200-300 msec) of slow repolarization term the plateau stage (25 in FIG. 1A) occurs. During the plateau it is not believed to be possible to initiate another action potential in the cell, because the sodium channels are inactivated.
Two mechanisms appear to be largely responsible for the long duration of the plateau, an inward current of calcium ions and an outward current of potassium ions. Both currents flow with their concentration gradients, across the membrane. The net result is that the two types of current electrically subtract from each other. In general, the flow of potassium and calcium is many times slower than the flow of the sodium, which is the reason why the plateau lasts so long. According to some theories, the potassium channels may also open as a result of the action potential, however, the probability of a potassium channel opening is dependent on the potential. Thus, many channels open only after the depolarization of the cell is under way or completed. Possibly, at least some of the potassium channels are activated by the calcium ions. In addition, some of the potassium channels are triggered by the repolarization of the membrane. The membrane permeability to potassium gradually increases, following its drop during the rapid depolarization (23). The calcium channels also conduct sodium back into the cell, which helps extend the plateau duration.
The inward calcium current during the normal cardiac action potential contributes to the action potential plateau and is also involved in the contractions (directly and/or indirectly) in the cardiac muscle cells. In a process termed calcium induced calcium release, the inward current of calcium induces the release of calcium ions stored in intracellular calcium stores (probably the sacroplasmic reticulum). The existence and importance of a physical link between the reticulum and the calcium channels in cardiac muscle is unclear. However, the response curve of these calcium stores may be bell-shaped, so that too great an influx of calcium may reduce the amount of available calcium relative to amount made available by a smaller influx.
In single cells and in groups of cells, time is required for cells to recover partial and full excitability during the repolarization process. While the cell is repolarizing (25, 27 in FIG. 1A), it enters a state of hyper polarization, during which the cell cannot be stimulated again to fire a new action potential. This state is called the refractory period. The refractory period is divided into two parts. During an absolute refractory period, the cell cannot be re-excited by an outside stimulus, regardless of the voltage level of the stimulus. During a relative refractory period, a much larger than usual stimulus signal is required to cause the cell to fire a new action potential. The refractory state is probably caused by the sodium channels requiring priming by a negative voltage, so the cell membrane cannot depolarize by flow of sodium ions until it is sufficiently repolarized. Once the cell returns to its resting potential (21), the cell may be depolarized again.
In an experimental methodology called voltage clamping, an electrical potential is maintained across at least a portion of a cell membrane to study the effects of voltage on ionic channels, ionic pumps and on the reactivity of the cell.
It is known that by applying a positive potential across the membrane, a cell may be made more sensitive to a depolarization signal. Some cells in the heart, such as the cells in the SA node (the natural pacemaker of the heart) have a resting potential of about −55 mV. As a result, their voltage-gated sodium channels are permanently inactivated and the depolarization stage (23) is slower than in ventricular cells (in general, the action potential of an SA node cell is different from that shown in FIG. 1A). However, cells in the SA node have a built-in leakage current, which causes a self-depolarization of the cell on a periodic basis. In general, it appears that when the potential of a cell stay below about −60 mV for a few msec, the voltage-gated sodium channels are blocked. Applying a negative potential across its membrane make a cell less sensitive to depolarization and also hyperpolarizes the cell membrane, which seems to reduce conduction velocity.
In modern cardiology many parameters of the heart's activation can be controlled. Pharmaceuticals can be used to control the conduction velocity, excitability, contractility and duration of the refractory periods in the heart. These pharmaceuticals may be used to treat arrhythmias and prevent fibrillations. A special kind of control can be achieved using a pacemaker. A pacemaker is an electronic device which is typically implanted to replace the heart's electrical excitation system or to bypass a blocked portion of the conduction system. In some types of pacemaker implantation, portions of the heart's conduction system, for example an atrial-ventricle (AV) node, must be ablated in order for the pacemaker to operate correctly.
Another type of cardiac electronic device is a defibrillator. As an end result of many diseases, the heart may become more susceptible to fibrillation, in which the activation of the heart is substantially random. A defibrillator senses this randomness and resets the heart by applying a high voltage impulse(s) to the heart.
Pharmaceuticals are generally limited in effectiveness in that they affect both healthy and diseased segments of the heart, usually, with a relatively low precision. Electronic pacemakers, are further limited in that they are invasive, generally require destruction of heart tissue and are not usually optimal in their effects. Defibrillators have substantially only one limitation. The act of defibrillation is very painful to the patient and traumatic to the heart.
“Electrical Stimulation of Cardiac Myoctes,” by Ravi Ranjan and Nitish V. Thakor, in Annals of Biomedical Engineering, Vol. 23, pp. 812-821, published by the Biomedical Engineering Society, 1995, the disclosure of which is incorporated herein by reference, describes several experiments in applying electric fields to cardiac muscle cells. These experiments were performed to test theories relating to electrical defibrillation, where each cell is exposed to different strengths and different relative orientations of electric fields. One result of these experiments was the discovery that if a defibrillation shock is applied during repolarization, the repolarization time is extended. In addition, it was reported that cells have a preferred polarization. Cardiac muscle cells tend to be more irregular at one end than at the other. It is theorized, in the article, that local “hot spots” of high electrical fields are generated at these irregularities and that these “hot spots” are the sites of initial depolarization within the cell, since it is at these sites that the threshold for depolarization is first reached. This theory also explains another result, namely that cells are more sensitive to electric fields in their longitudinal direction than in their transverse direction, since the irregularities are concentrated at the cell ends. In addition, the asymmetric irregularity of the cells may explain results which showed a preferred polarity of the applied electric field.
The electrical activation of skeleton muscle cells is similar to that of cardiac cells in that a depolarization event induces contraction of muscle fibers. However, skeleton muscle is divided into isolated muscle bundles, each of which is individually enervated by action potential generating nerve cells. Thus, the effect of an action potential is local, while in a cardiac muscle, where all the muscle cells are electrically connected, an action potential is transmitted to the entire heart from a single loci of action potential generation. In addition, the chemical aspects of activation of skeletal muscle is somewhat different from those of cardiac muscle.
“Muscle Recruitment with Infrafascicular Electrodes”, by Nicola Nannini and Kenneth Horch, IEEE Transactions on Biomedical Engineering, Vol. 38, No. 8, pp. 769-776, August 1991, the disclosure of which is incorporated herein by reference, describes a method of varying the contractile force of skeletal muscles, by “recruiting” a varying number of muscle fibers. In recruiting, the contractile force of a muscle is determined by the number of muscle fibers which are activated by a stimulus.
However, it is generally accepted that cardiac muscle fibers function as a syncytium such that each and every cell contracts at each beat. Thus, there are no cardiac muscles fibers available for recruitment. See for example, “Excitation Contraction Coupling and Cardiac Contractile Force”, by Donald M. Bers, Chapter 2, page 17, Kluwer Academic, 1991, the disclosure of which is incorporated herein by reference. This citation also states that in cardiac muscle cells, contractile force is varied in large part by changes in peak calcium.
“Effect of Field Stimulation on Cellular Repolarization in Rabbit Myocardium”, by Stephen B. Knisley, William M. Smith and Raymond E. Ideker, Circulation Research, Vol. 70, No. 4, pp. 707-715, April 1992, the disclosure of which is incorporated herein by reference, describes the effect of an electrical field on rabbit myocardium. In particular, this article describes prolongation of an action potential as a result of a defibrillation shock and ways by which this effect can cause defibrillation to fail. One hypothesis is that defibrillation affects cardiac cells by exciting certain cells which are relatively less refractory than others and causes the excited cells to generate a new action potential, effectively increasing the depolarization time.
“Optical Recording in the Rabbit Heart Show That Defibrillation Strength Shocks Prolong the Duration of Depolarization and the Refractory Period”, by Stephen M. Dillon, Circulation Research, Vol. 69, No. 3, pp. 842-856, September, 1991, the disclosure of which is incorporated herein by reference, explains the effect of prolonged repolarization as caused by the generation of a new action potential in what was thought to be refractory tissue as a result of the defibrillation shock. This article also proves experimentally that such an electric shock does not damage the cardiac muscle tissue and that the effect of a second action potential is not due to recruitment of previously unactivated muscle fibers. It is hypothesized in this article that the shocks hyperpolarize portions of the cellular membrane and thus reactivate the sodium channels. In the experiments described in this article, the activity of calcium channels is blocked by the application of methoxy-verapamil.
“Electrical Resistances of Interstitial and Microvascular Space as Determinants of the Extracellular Electrical field and Velocity of Propagation in Ventricular Myocardium”, by Johannes Fleischhauer, Lilly Lehmann and Andre G. Kleber, Circulation, Vol. 92, No. 3, pp. 587-594, Aug. 1, 1995, the disclosure of which is incorporated herein by reference, describes electrical conduction characteristics of cardiac muscle.
“Inhomogeneity of Cellular Activation Time and Vmax in Normal Myocardial Tissue Under Electrical Field Stimulation”, by Akihiko Taniguchi, Junji Toyama, Itsuo Kodama, Takafumi Anno, Masaki Shirakawa and Shiro Usui, American Journal of Physiology, Vol. 267 (Heart Circulation Physiology, Vol. 36), pp. H694-H705, 1994, the disclosure of which is incorporated herein by reference, describes various interactions between electro-tonic currents and action potential upstrokes.
“Effect of Light on Calcium Transport in Bull Sperm Cells”, by R. Lubart, H. Friedmann, T. Levinshal, R. Lavie and H. Breitbart, Journal of Photochemical Photobiology B, Vol. 14, No. 4, pp. 337-341, Sep. 12, 1992, the disclosure of which is incorporated herein by reference, describes an effect of light on bull sperm cells, in which laser light increases the calcium transport in these cells. It is also known that low level laser light affects calcium transport in other types of cells, for example as described in U.S. Pat. No. 5,464,436, the disclosure of which is incorporated herein by reference.
The ability of electro-magnetic radiation to affect calcium transport in cardiac myocytes is well documented. Loginov V A, “Accumulation of Calcium Ions in Myocardial Sarcoplasmic Reticulum of Restrained Rats Exposed to the Pulsed Electromagnetic Field”, in Aviakosm Ekolog Med, Vol. 26, No. 2, pp. 49-51, March-April, 1992, the disclosure of which is incorporated herein by reference, describes an experiment in which rats were exposed to a 1 Hz field of between 6 and 24 mTesla. After one month, a reduction of 33 percent in the velocity of calcium accumulation was observed. After a second month, the accumulation velocity was back to normal, probably due to an adaptation mechanism.
Schwartz J L, House D E and Mealing G A, in “Exposure of Frog Hearts to CW or Amplitude-Modulated VHP Fields: Selective Efflux of Calcium Ions at 16 Hz”, Bioelectromagnetics, Vol. 11, No. 4, pp. 349-358, 1990, the disclosure of which is incorporated herein by reference, describes an experiment in which the efflux of calcium ions in isolated frog hearts was increased by between 18 and 21% by the application of a 16 Hz modulated VHF electromagnetic field.
Lindstrom E, Lindstrom P, Berglund A, Lundgren E and Mild K H, in “Intracellular Calcium Oscillations in a T-cell Line After Exposure to Extremely-Low-Frequency Magnetic Fields with Variable Frequencies and Flux Densities”, Bioelectromagnetics, Vol. 16, No. 1, pp. 41-47, 1995, the disclosure of which is incorporated herein by reference, describes an experiment in which magnetic fields, at frequency between 5 and 100 Hz (Peak at 50 Hz) and with intensities of between 0.04 and 0.15 mTesla affected calcium ion transport in T-cells.
Loginov V A, Gorbatenkova N V and Klimovitskii Vla, in “Effects of an Impulse Electromagnetic Field on Calcium Ion Accumulation in the Sarcoplasmatic Reticulum of the Rat Myocardium”, Kosm Biol Aviakosm Med, Vol. 25, No. 5, pp. 51-53, September-October, 1991, the disclosure of which is incorporated herein by reference, describes an experiment in which a 100 minute exposure to a 1 msec impulse, 10 Hz frequency and 1-10 mTesla field produced a 70% inhibition of calcium transfer across the sarcoplasmic reticulum. The effect is hypothesized to be associated with direct inhibition of Ca-ATPase.
It should be noted that some researchers claim that low frequency magnetic fields do NOT have the above reported effects. For example, Coulton L A and Barker A T, in “Magnetic Fields and Intracellular Calcium: Effects on Lymphocytes Exposed to Conditions for ‘Cyclotron Resonance’”, Phys Med Biol, Vol. 38, No. 3, pp. 347-360, March, 1993, the disclosure of which is incorporated herein by references, exposed lymphocytes to radiation at 16 and 50 Hz, for a duration of 60 minutes and failed to detect any changes in calcium concentration.
Pumir A, Plaza F and Krinsky V I, in “Control of Rotating Waves in Cardiac Muscle: Analysis of the Effect of Electric Fields”, Proc R Soc Lond B Biol Sci, Vol. 257, No. 1349, pp. 129-34, Aug. 22, 1994, the disclosure of which is incorporated herein by reference, describes that an application of an external electric field to cardiac muscle affects conduction velocity by a few percent. This effect is due to the hyperpolarization of one end of muscle cells and a depolarization of the other end of the cell. In particular, an externally applied electric field favors propagation antiparallel to it. It is suggested in the article to use this effect on conduction velocity to treat arrhythmias by urging rotating waves, which are the precursors to arrhythmias, to drift sideways to non-excitable tissue and die.
“Control of Muscle Contractile Force Through Indirect High-Frequency Stimulation”, by M. Sblomonow, E. Eldred, J. Lyman and J. Foster, American Journal of Physical Medicine, Vol. 62, No. 2, pp. 71-82, April 1983, the disclosure of which is incorporated herein by reference, describes a method of controlling skeletal muscle contraction by varying various parameters of a 500 Hz pulse of electrical stimulation to the muscle.
“Biomedical Engineering Handbook”, ed. Joseph D. Bronzino, chapter 82.4, page 1288, IEEE press/CRC press, 1995, describes the use of precisely timed subthreshold stimuli, simultaneous stimulation at multiple sites and pacing with elevated energies at the site of a tachycardia foci, to prevent tachycardia. However, none of these methods had proven practical at the time the book was written. In addition a biphasic defibrillation scheme is described and it is theorized that biphasic defibrillation schemes are more effective by virtue of a larger voltage change when the phase changes or by the biphasic waveform causing hyperpolarization of tissue and reactivation of sodium channels.
“Subthreshold Conditioning Stimuli Prolong Human Ventricular Refractoriness”, Windle J R, Miles W M, Zipes D P and Prystowsky E N, American Journal of Cardiology, Vol. 57, No. 6, pp. 381-386, February, 1986, the disclosure of which is incorporated herein by reference, describes a study in which subthreshold stimuli were applied before a premature stimulus and effectively blocked the premature stimulus from having a pro-arrhythmic effect by a mechanism of increasing the refractory period of right ventricular heart tissue.
“Ultrarapid Subthreshold Stimulation for Termination of Atrioventricular Node Reentrant Tachycardia”, Fromer M and Shenasa M, Journal of the American Collage of Cardiology, Vol. 20, No. 4, pp. 879-883, October, 1992, the disclosure of which is incorporated herein by reference, describes a study in which trains of subthreshold stimuli were applied asynchronously to an area near a reentry circuit and thereby terminated the arrhythmia. Subthreshold stimuli were described as having both an inhibitory and a facilitating effect on conduction. In addition, subthreshold stimuli are described as reducing the threshold of excitability, possibly even causing an action potential.
“Inhibition of Premature Ventricular Extrastimuli by Subthreshold Conditioning Stimuli”, Skale B, Kaliok M J, Prystowsky E N, Gill R M and Zipes D P, Journal of the American Collage of Cardiology, Vol. 6, No. 1, pp. 133-140, July, 1985, the disclosure of which is incorporated herein by reference, describes an animal study in which a train of 1 msec duration pulses were applied to a ventricle 2 msec before a premature stimuli, inhibited the response to the premature stimuli, with a high frequency train delaying the response for a much longer amount of time (152 msec) than a single pulse (20 msec). The delay between the pacing of the ventricle and the pulse train was 75 msec. However, the subthreshold stimuli only had this effect when delivered to the same site as the premature stimulus. It is suggested to use a subthreshold stimuli in to prevent or terminate tachycardias, however, it is noted that this suggestion is restrained by the spatial limitation of the technique.
“The Phase of Supernormal Excitation in Relation to the Strength of Subthreshold Stimuli”, Yokoyama M, Japanese Heart Journal, Vol. 17, No. 3, pp. 35-325, May, 1976, the disclosure of which is incorporated herein by reference, describes the effect of varying the amplitude of a subthreshold stimuli on supernormal excitation. When the amplitude of the stimuli was increased, the supernormal excitation phase increased in length.